On Fri, 28 Jul 2006 10:22:21 -0700, wrote:
1) The solstice occurs when the sun reaches its maximum declination of
the year, which should be around 23.5 degrees N. The declination
never decreases between the vernal equinox and the summer solstice,
and never increases between the summer solstice and the autumnal
equinox.

Correct, for half the year. You can include the whoile year by going
solstice-to-solstice: the Sun increases in dec from winter solstice to
summer solstice, decreases from summer solstice to winter solstice.
The appropriate equinoxes occur in between.

2) The sun's RA at the solstice should be very close to 6 hours, but
since the RA is defined by the vernal equinox rather than the
solstice, the solstice may not occur at exactly 6 hours.

True for times off a chart's given epoch. By definition though, 0h is
the intersection of the equator and the ecliptic, and the
intersections of the ecliptic with the 6, 18, and 12 hour circles mark
the equioxes and solstices.

3) The sun's RA and Dec do not depend on my location, so if I read in
an almanac that the solstice occurred on June 21 at 8:26 AM EDT, I
need only correct for my time zone, rather than fractions of a time
zone (as I would if I were trying to calculate my local sunrise, for
example).

The RA and dec do depend on your location. However, given the diameter
of the Earth, it's really, really, miniscule. The solsitces and
equinoxes take the alignments of the centers of the bodies. Your
parallax off of that will vary.

4) SNP has two sets of RA/Dec coordinates in its info panel, one
labeled J2000, and one labeled JNow. I assume that JNow will be more
accurate for current observations.

Those are the epochs. The great circle equator rotates around the sky
as the earth's axis wobbles over 25,600 years. That's enough to cause
errors when using large magnifications in telescopes. Typically,
standard epocjs are given every 50 years. 1950 was a common epoch
until halfway to 2000 (1975), when it became less accurate, but more
available because of already-preinted charts. Software can now display
wahtever epoch you like, down to real-time (now).

OK, assuming all that is correct, here is what I found. All times are
EDT.

1) On June 21, the maximum RA reached was 23 degrees and 26.386
minutes. This was maintained from 2:00 PM to 3:16 PM EDT. The
almanac says the solstice should have been at 8:26 AM EDT.

2) When I ran the time backward from there, the declination slowly
decreased, but it hit a minimum (when it was 23deg 26.314') that
lasted from 2:03 AM to 3:39 AM of June 21, then it began to increase
as I went farther back. It peaked at 23deg 26.332' from about 7:37 PM
to 8:20 PM of June 20, and steadily decreased as I went earlier than
that..

So if SNP is correct, there were three solstices, i.e. a max on both
the 20th and 21st, and a local min between them. I could understand
if round-off errors produced fluctuations right around the true
solstice, but I can't understand
a) the absolute max being nearly six hours off the published time,
and
b) apparently smooth progressions between two maximums nearly a day
apart.

Any explanations, or pointers to URLs, appreciated.

Dunno offhand. Difference in epochs?

=============
- Dale Gombert (SkySea at aol.com)
122.38W, 47.58N, W. Seattle, WA
http://flavorj.com/~skysea